Methods for producing optical fiber by focusing high viscosity liquid

ABSTRACT

The invention is directed to the production of optical fibers from optical fiber preforms using flow physics. The present methods provide for the “drawing” of an optical fiber preform using focusing of the preform by a surrounding fluid, e.g. a heated gas.

CROSS-REFERENCES

[0001] This application is a continuation-in-part our earlier filedprovisional patent application Ser. No. 60/188,310 filed Mar. 10, 2000;Ser. No. 60/188,299 filed on Mar. 10, 2000; Ser. No. 60/249,665 filedNov. 17, 2000; and Ser. No. 60/249,848 filed Nov. 17, 2000 whichapplications are incorporated herein by reference and to whichapplications is claimed priority.

FIELD OF THE INVENTION

[0002] This application generally relates to the field of producingelongated strands from highly viscous liquid materials, and moreparticularly to the creation of optical fibers from molten glass usingfocused fluid technology.

BACKGROUND OF THE INVENTION

[0003] Optical fibers are thin strands of materials, such as glass orpolymeric compounds, capable of transmitting an optical signalcontaining a large amount of information over long distances with verylow loss. (See U.S. Pat. Nos. 6,128,429; 6,098,428; 6,057,034 andpublications and patents cited in each of these patents) Opticalcommunication systems based on glass optical fibers allow communicationsignals to be transmitted over long distances with low attention and atextremely high data rates, or band width capacity. This capabilityarises form the propagation of a single optical signal mode in the lowloss windows of glass located at the near-infrared wavelengths. Sincethe introduction of erbium doped fiber amplifier (EDFA), the last decadehas witnessed the emergence of the glass optical fiber as the standarddata transmission medium for wide area networks (WANs).

[0004] Conventional optical fibers are typically manufactured byconstructing an optical fiber preform of appropriate composition anddrawing a fiber from the preform. (See U.S. Pat. No. 6,053,012 andpatents and publications cited therein) A typical preform is aconcentric glass rod having a length of about one meter and an outerdiameter of 20-200 mm. The inner core of the rod is a high purity, lowloss glass such as germanium silicate glass having a diameter of about1-5 mm. The concentric outer cylinder, referred to as cladding, is alayer of glass with a lower index of refraction than the inner core.

[0005] In the conventional manufacture of an optical fiber, the preformis lowered into the insulated susceptor of an RF induction furnace whereit is heated to a high drawing temperature. (See U.S. Pat. Nos.5,741,384; 5,698,124 and patents and publications cited in each) Astrand of glass is pulled from the heated portion of the preform at atension and rate to draw a fiber of desired diameter. One of the primarydifficulties with this conventional process is contamination of thefiber from the materials of the induction furnace. Even very smallparticulates from the insulation or susceptor can produce localized weakpoints in the fiber which will ultimately result in breakage or otherforms of failure. U.S. Pat. No. 4,440,556 describes an early attempt tosolve this contamination problem by directing a plasma torch axiallyonto a preform and drawing a fiber axially through a central passage inthe torch. The difficulty with this approach is that to reach thecentral passage, the drawn fiber must pass through the plasma fireball.But plasma shapes are notoriously difficult to control, and even minorfluctuations in shape can subject the delicate drawn fiber to severetemperature fluctuations.

[0006] Another difficulty arises from the use of increasingly largerdiameter preforms. With larger diameter preforms it is very difficult togenerate a sufficiently large plasma fireball to cover the entirediameter of the preform. The result is non-uniform heating in thedrawing region. Similar methods, such as the method described in U.S.Pat. No. 5,672,192, address some of the problems inherent in thesemethods, but still requires the use of a plasma torch and thus has manyof the limitations inherent to this use.

[0007] The success of the single-mode glass optical fiber incommunication backbones has given rise to the concept of opticalnetworking. These networks serve to integrate data streams over alloptical systems as communication signals make their way from WANs downto smaller local area networks (LANs) and eventually to the end user byfiber to the desktop. The increased use of optical networks, based inlarge part on the recent explosion of the Internet and use of the WorldWide Web, has demanded vastly higher bandwidth performance in short- andmedium-based applications.

[0008] There is thus a need in the art for improved methods of producingglass optical fibers to meet the growing demands of consumer use. Inaddition, there is a growing demand for better optical fibers, bothsingle mode and multimode optical fibers.

SUMMARY OF THE INVENTION

[0009] The invention is directed to the production of optical fibersusing flow physics. The present methods provide for the focusedextrusion of a highly viscous material such as molten germanium silicateglass, either directly from a molten liquid or from a perform, using afluid (e.g. a heated gas or liquid) that surrounds and focuses the highviscosity liquid stream or preform, resulting in an evenly shaped,elongated fiber. The invention also provides methods and devices for themanufacture of optical performs, which can then be drawn usingconventional technology or using the drawing methods disclosed herein.

[0010] A flow physics methodology which is applied to low viscosityfluids is described in publications such as U.S. Pat. No. 6,116,516issued Sep. 12, 2000; U.S. Pat. No. 6,187,214 issued Sep. 13, 2001; U.S.Pat. No. 6,197,835 issued Mar. 6, 2001; and U.S. Pat. No. 6,196,525issued Mar. 6, 2001. However, these disclosures relate to the extrusionof low viscosity fluids. What is mean by low viscosity fluid is that thefluid has a Reynolds number which is relatively high, for example anumber about 10 or more. The extrusion of low viscosity fluids iscarried out under conditions using forces which are not dominated by theviscosity of the fluid but rather dominated by the mass of the fluid orits density. By analogy, the engine of car moves the car forward usingthe power of the engine largely to have an effect on the mass of the carand, to a lesser extent, in order to overcome the frictional resistancesexisting between various components. However, if the frictional forcesare substantially increased such as by applying the emergency brake ofthe car then there frictional forces become the dominant forces whichmust be overcome in order to move the car forward. In this analogy thefrictional forces relate to the viscosity of the fluid.

[0011] The disclosure provided here is directed towards methodologywhich describes creating streams and fibers from high viscosity fluids.The term “high viscosity fluid” is intended to encompass fluids whereinthe Reynolds number is relatively small, specifically a Reynolds numberof about 1 or less. More particularly, the Reynolds number in a veryhigh viscosity fluid is less than about 0.1. With high viscosity fluids,as with the car with the emergency brake on, the viscosity of the fluidbecomes a dominating factor in terms of what must be overcome by theforces applied in order to move the fluid forward just as the frictionalresistance created by the emergency brake becomes the dominating factorwhich the car engine must overcome in order to move the car forward.

[0012] A section entitled “mathematical formulation” is included below.This section includes equations which will be understood by thoseskilled in the art upon reading this disclosure as applicable to themanufacturing of streams and fibers from high viscosity fluids such asthe high viscosity fluid of molten silica glass with a high viscosityfluid of a heated glass preform used in creating fibers which are usedto optically transmit information.

[0013] In a first embodiment, elongated fibers such as optical fibersare produced directly from a highly viscous liquid, e.g., moltensilicate glass, by subjecting a stream of the viscous liquid to asurrounding, focusing fluid. This allows fibers to be generated withoutthe need for producing a perform, and can also allow the extrusion ofmultiple fibers simultaneously. This extrusion is particularlyadvantageous in that the fiber stream does not contact the surface ofthe orifice upon extrusion of the fiber from the devices of theinvention because the extruded fiber is completely surrounded by andfocused with the focusing fluid which may be a gas. This makes itpossible to reduce contamination of the fiber and essentially preventsclogging of the device orifice. Elongated fibers produced can have anydesired diameter but are preferably 200 microns or less in diameter andmay be from 1 micron to 50 microns in diameter.

[0014] In another embodiment, optical fiber preforms are reduced indiameter and increased in length using the focusing properties of asurrounding fluid. The optical fiber precursors (i.e. the preforms) areheated to a temperature that allows the preform material to maintain thebasic structural elements of the preforms while allowing the preform tobecome ductile or specifically to be stretched to the desired length andlateral dimensions, i.e. a temperature which renders the optical fiberprecursor ductile and allows the fiber to maintain the lateralrelationship of the preform. The focusing process may be repeated toprovide the desired diameter and/or length of the fiber, a focusingfluid and the narrowed structure can be further narrowed by repeatedexposure to focusing fluid.

[0015] In another embodiment hollow fibers are produced. The hollowfibers are extruded from a source comprised of two concentricallypositioned tubes. The center tube extrudes a gas such as air or a highlypure inert gas and the surrounding concentric tube extrudes moltensilicate glass. The extruded silicate glass forms a hollow tube and isfocused to a jet by the surrounding flow of gas in a pressure chamber.Multiple hollow fibers may be extruded simultaneously and joinedtogether before solidifying, e.g. to form a photonic band gap structure.

[0016] An advantage of the invention is that the focusing pressure fromthe surrounding focusing fluid provides pressure distribution on theviscous liquid extruded or the preform and the pressure distribution canbe calculated mathematically to show that it suppresses instabilitybefore any fiber drawing viscosity thereby indicating a theoreticallyunlimited increase in productivity.

[0017] Another advantage of the invention is that shear stress on thefiber produced from the extruded viscous material can be reduced to aminimum thereby allowing the controlled production of complex fiberstructures including hollow fibers which can be combined to produce anydesired configuration of photonic bandgap structures.

[0018] Yet another advantage of the invention is that gas temperaturedistribution along the nozzle is very rapidly transferred to the drawnfiber material thereby providing a means for a simple and acceleratedcontrol of the fiber temperature profile and offering a robust andsimple manner of controlling the fiber quenching process and enhancementof fiber quality.

[0019] An advantage of the invention is that the optical fibers formedare uniform in size and shape along this length and are created with arelatively small amount of energy.

[0020] Another advantage of the invention is that it allows multiplefiber extrusions to take place simultaneously, thus allowing the fibersto be extruded as a bundle.

[0021] Yet another advantage of the invention is that the fibers can beextruded as a coated fiber using concentric needles in the devices ofthe invention.

[0022] Yet another advantage of the invention is that optical fibers canbe created without contamination, resulting in optical fibers withoutlocalized weak points in the fiber caused by such contamination.

[0023] Yet another advantage of the invention is that fiber forming andstability using the production methods of the invention can be enhancedusing an appropriate external pressure distribution.

[0024] Yet another advantage of the invention is that fiber stress canbe dramatically reduced upon extrusion of the devices of the invention,as glass to solid contact is avoided due to the extrusion of the glasssurrounded by the focusing gas or liquid.

[0025] Yet another advantage of the invention is that the device of theinvention will have minimal contamination and/or clogging from theextrusion of the fiber, as the exit orifice never touches the fluid orperform.

[0026] Yet another advantage is that fiber quality is enhances by rapidfiber quenching owing to the coflowing gas expansion.

[0027] Yet another advantage of the invention is that complex fiberconcentric structures can be formed by the dramatic reduction of radialviscous stresses of the present methods as compared to classictechniques.

[0028] Yet another advantage of the present invention is that whenpreforms are used they are not subject to fluctuations in shape based onthe focusing procedure, and thus the drawn fibers are not subject tosevere temperature fluctuations as with the use of plasma fireballs.

[0029] Yet another advantage is that the extrusion methods can bedesigned to create fibers with discrete functional elements based on theorientation of extrusion. This allows the production of specializedfiber structures, such as photonic bandgap structures, in conventionallength fibers.

[0030] Yet another advantage is that the present methods can be usedwith preforms having very distinct structural elements, since theintegrity of the relationship of the structural elements is maintainedin the focusing procedure.

[0031] Yet another advantage is that the present methods can be usedwith larger diameter preforms.

[0032] These and other aspects, objects, features and advantages willbecome apparent to those skilled in the art upon reading this disclosurein combination with the figures provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a schematic cross-sectional view of the embodiment ofthe present invention wherein the source material is a glass preformwhich is heated in an oven and extruded through a nozzle.

[0034]FIG. 2 is a cross-sectional view of a particular configuration ofa nozzle component used in the production of streams and fibers fromhigh viscosity fluids in accordance with the present invention.

[0035]FIG. 3 includes graphs 3A, 3B and 3C wherein graph 3A shows thatfor a given λ different nozzle configurations are constructed in orderto provide a stable stream and fiber when extruding a high viscosityfluid wherein FIG. 3B shows a nozzle configuration where λ=2 and FIG. 3Cshows the nozzle configuration when λ=6.

[0036]FIG. 4 is a graph which shows the pressure needed in order toobtain a stable jet with different nozzle configurations based ondifferent λ values.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] Before the present fiber extrusion device and method aredescribed, it is to be understood that this invention is not limited tothe particular components and steps described, as such may, of course,vary. It is also to be understood that the terminology used herein isfor the purpose of describing particular embodiments only, and is notintended to be limiting, since the scope of the present invention willbe limited only by the appended claims.

[0038] It must be noted that as used herein and in the appended claims,the singular forms “a”, “and,” and “the” include plural referents unlessthe context clearly dictates otherwise. Thus, for example, reference to“a dopant” includes a plurality of dopants and reference to “the fluid ”includes reference to a mixture of fluids, and equivalents thereof knownto those skilled in the art, and so forth.

[0039] Unless defined otherwise, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this invention belongs. Although any methodsand materials similar or equivalent to those described herein can beused in the practice or testing of the present invention, the preferredmethods and materials are now described. All publications mentionedherein are incorporated herein by reference to disclose and describe themethods and/or materials in connection with which the publications arecited.

[0040] The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.

Definitions

[0041] The term “ductile” as used herein with reference to certainmaterials refers to materials in a phase that allows the materials to bedrawn out into a strand. Ductile materials as used herein morepreferably refers to materials for the creation of an optical fiber orpreform that are solid at room temperature (e.g. silicate glass) but aremore easily shaped or drawn into an elongated fiber at elevatedtemperatures. The term as used herein includes materials in a flowable,or ductile, or heated (or otherwise treated) form.

[0042] The terms “drawing” and “drawn” as used herein refer to theprocess of elongating a stream of material to create an elongated fiber.The drawing using flow physics results in a fiber that is consistentlysized (in size and shape of cross-section along its length) and hassignificantly reduced lateral dimensions (cross-section) as compared tothe original liquid stream or preform from which it is drawn. In adrawing process the material in the center of a solid cylinder is drawnaway from the cylinder and thereby takes on a narrower diameter ascompared to the cylinder. The length is increased and the diameterdecreased.

[0043] The term “focusing fluid” is the fluid used to focus the liquidstream or the preform. This fluid is preferably a heated gas, althoughthe fluid may also be a liquid (with the same or preferably low densityas compared to the liquid stream being focused) that is immiscible withthe liquid stream or the ductile preform.

[0044] The term “preform” as used herein refers to a structure that is aprecursor to an optical fiber e.g. a solid cylinder of silica glass. Apreform has the basic structural elements of an optical fiber (but has alarger diameter), and is generally heated and drawn into the narrowerelongated configuration of an optical fiber. Preforms can be made ofnumerous different materials, as is known in the art, including glass(e.g. silicate), plastic, graphite and the like. In a preferredembodiment, the preform may be comprised of one or more photonic bandgapstructures (e.g. a group of hollow cylinders) that allow light to travelthrough a hollow portion of the fiber that is surrounded by the photonicbandgap structure.

[0045] The term “high viscosity fluid” and “high viscosity liquid” meansa flowable material which has a viscosity substantially greater thanwater i.e. 5 times or more the viscosity of water. Specifically, theterm “high viscosity liquid” is intended to encompass fluids wherein theReynolds number is relatively small, specifically about 1 or less andmore preferably 0.1 or less. Preferred high viscosity fluids includemolten silica glass and molten glass in various doped forms which areused to produce fiber in order to transmit information optically.

Device in General

[0046] Referring to FIG. 1 there is shown a schematic cross-sectionalview of the invention. In this particular configuration the source 1 ofthe high viscosity fluid is a glass preform. At least the end portion 2of the preform 1 is included within an oven 3 or the end to the preformis heated. Further, a focusing fluid such as air or more preferably aheated inert gas is provided into the oven in order to provide apressure P⁰. The only exit for the focusing fluid in the pressurizedoven is out of an opening 4 created via a nozzle 5. As the gas in theoven rushes out of the opening 4 through the nozzle 5 the high viscosityfluid is pulled through the nozzle 5 eventually exiting the opening 4and creating an elongated fiber 6. This particular embodiment is aschematic embodiment and is merely provided as exemplary in order toconvey the essence of the invention which is the formation of a stablefiber using a focusing fluid to draw out a high viscosity liquid. Thoseskilled in the art reading this disclosure will understand thatvariabilities will include factors such as the temperature and pressureof the oven 3, the shape of the nozzle 5, the length of the nozzle andthe viscosity of the high viscosity fluid being extruded through thenozzle.

[0047] In an alternative embodiment the preform is replaced with ahollow tube which is preferably a metal tube which is continuouslysupplied with high viscosity fluid such as molten silica glass. Both ofthese embodiments, i.e. the preform embodiment and the molten liquidsupplied from a tube opening have variations which make it possible toproduce hollow fibers. Specifically, the preform can be a preform whichis solid as is shown in FIG. 1 but also may be a preform comprised of ahollow tube which is elongated and extruded using the same methodologywith the ultimate fiber extruded out of the opening 4 being a hollowfiber. When the molten silica is supplied by a cylindrical tube thattube has a second tube concentrically positioned within it whichsupplies a gas which is preferably a heated inert gas. A heated inertgas must be supplied at a sufficiently high pressure so that the hollowfiber is not collapsed in the oven 3. Thus, the pressure within thehollow tube is balanced so that it is substantially equal to thepressure inside the oven 3 so that the hollow tube is not collapsed butrather is extruded out of the opening 4 to form a hollow fiber. In allof the embodiments is it preferable that the pressure P₀ inside of theoven be sufficiently high and the configuration of the nozzle be suchthat the extrusion of gas out of the opening 4 is supersonic i.e. fasterthan the speed of sound.

[0048] Referring now to FIG. 2 there is shown a schematiccross-sectional view of a nozzle of the invention. The particularconfiguration of the nozzle is important in order to obtain desiredresults which particularly include the extrusion of the fiber in astable manner out of the opening 4 so that the fiber does not flap ormove excessively resulting in breakage or non-uniformity of the fibermaterial being extruded. The geometry of the internal surface 7 of thenozzle 5 can be determined by the following formula:

P(x)=p ₀ e ^(−λx)

[0049] P₀ is the internal pressure inside the oven 3; λ is a constantwherein λ is greater than 0.635 in order to obtain supersonic expansionand x is a function.

[0050] As indicated in FIG. 2, for the shown configuration λ is 5.65; P₀is greater than or equal to 0.325 for all E greater than or equal to 0.Such will result in absolute stability for drawing of a high viscosityfluid in accordance with the following equation:$P_{0} \geq \frac{\mu_{l}V_{1}}{L}$

[0051] In the above formula μ₁ is the viscosity of the high viscosityliquid; V₁ is the velocity of the fiber inside of the nozzle which issubstantially greater than the velocity V₀ of the fiber as it extrudesoff of the end of the preform or out of the tube; and L is the length ofthe nozzle and therefore the length over which the focusing fluid suchas the gas provides substantial energy to the fiber pulling, forcing ordrawing it forward out of the opening 4.

[0052] Referring now to FIG. 3 which includes graphs 3A, 3B and 3C itcan be seen that different λ provide different nozzle configurationswhich can result in absolute stability of the fiber drawn out of theopening 4 of the nozzle. Specifically, within FIG. 3A plots are drawnfor λ=2, 4, 6 and 10. Within FIG. 3B the nozzle configuration is shownfor λ=2 and within FIG. 3C a nozzle configuration is shown for λ=6.

[0053] Referring now to FIG. 4 an additional parameter which is thepressure is taken into consideration. For a given λ which is plotted onthe λ axis the graph in FIG. 4 shows the amount of pressure at theentrance to the oven which is needed in order to obtain a stable jet.

Forces Exerted for High Viscous Fluids

[0054] A model for the production of a glass fiber takes into account anumber of different parameters. The parameter window used (i.e. the setof special values for the properties such as flow-rate used, feedingneedle diameter, orifice diameter, pressure ratio, etc.) should be largeenough to be compatible with virtually any desired viscous liquid orpreform (dynamic viscosities in the range from 10⁻⁴ to 1 kg m⁻¹s⁻¹).

[0055] When the preform-fluid interface is created, the preform thatemerges from the outlet of the feeding point is concentrically withdrawninto the nozzle. After the preform emerges from the exit port, it isaccelerated by tangential sweeping forces exerted by the focusing fluid(e.g. gas stream) flowing on its surface, which gradually decreases thepreform cross-section dimensions. Stated differently the gas flow actsas a lens and focuses the preform as it moves toward and into the exitorifice of the pressure chamber. This is schematically shown in FIG. 1.

[0056] The forces exerted by the fluid flow on the preform surfaceshould be steady enough to prevent irregular surface oscillations.Therefore, any turbulence in the gas motion should be avoided; even ifthe gas velocity is high, the characteristic size of the orifice shouldensure that the gas motion is laminar (similar to the boundary layersformed on the jet and on the inner surface of the nozzle or hole).

[0057] One of the advantages of the invention is that a desired coolingeffect can be obtained on the surface of the fiber 6 as it exits theopening 4 of the nozzle 5. Specifically, as the gas exits the opening 4of the nozzle the gas rapidly expands making it possible to absorbenergy or the heat on the surface of the fiber 6. This makes it possibleto quickly cool the fiber which fiber may be at a substantially moltenstate when exiting the opening 4. The cooled fiber is then solidifiedand can be moved into storage. As shown in FIG. 1 the oven ispressurized with a pressure P₀ and has a single nozzle with a singleopening 4. However, the invention contemplates an embodiment where theoven or pressurized area includes multiple nozzles 5 with multipleopenings 4 each being supplied by a different high viscosity supplysource which are each positioned upstream of the opening 4 in each ofthe nozzles 5. In such a configuration only a single oven or heatingelement may be required. Further, because the gas or pressure within theoven or pressure chamber is sufficient to focus and move the fiberthrough the nozzle precise positioning of the nozzle is not crucialprovided the nozzle and its opening to the outer atmosphere ispositioned substantially downstream of the flow of the high viscosityfluid.

[0058] Using the embodiment as shown in FIG. 1 it is possible to providea coated fiber by including a coating a material inside the oven 3. Anydesired coating or cladding material could be included within the gas orother focusing fluid material provided to the high viscosity fluid.Still further, in the embodiment where the high viscosity fluid isprovided to the oven by a tube and that tube encompasses aconcentrically positioned tube which extrudes a gas that gas can includea coating or cladding material which can be used to coat or clad theinside of the fiber being created. Those skilled in the art willcontemplate a range of different materials which are desirably coatedonto the inside and/or outside of the fiber in order to provide desiredoptical characteristics or other characteristics to the fiber beingproduced.

Optical Fiber Preforms

[0059] Optical fibers are typically manufactured by constructing anoptical fiber preform of appropriate composition and drawing a fiberfrom the preform. The preform is constructed and then subjected to ahigh temperature drawing procedure where the center of the solid preformis pulled away thereby increasing the length of the fiber whilesimultaneously decreasing the lateral dimensions of the fiber. A typicalpreform is a concentric glass rod having a length of about one meter andan outer diameter of 20-200 mm. The inner core of the rod is a highpurity, low loss glass having a diameter of about 1-5 mm, and the glassis optionally doped for increased optical performance. The concentricouter cylinder, referred to as cladding, is a layer of glass with alower index of refraction than the inner core.

[0060] In one particular embodiment, the present invention can be usedto construct elements (tubes and rods) for the construction of opticalfibers and/or optical fiber preforms. The present invention can be usedto form single rods having an inner core and an outer cladding core. Thecore, for example may comprises silica doped with oxides of germanium orphosphorous or, alternatively, the fibers may comprise a polymer-cladsilica glass core. The cladding can be a pure or doped silicate such asfluorosilicate, an organosiloxane such as polydimethylsiloxane or afluorinated acrylic polymer. See e.g., U.S. Pat. No. 6,014,488. Thefibers may also contain a third, outer coating, e.g. a coating with aresin containing a pigment to allow color coating of a fiber.

[0061] The two layers (or more) layers of the rod are extruded ascylindrical tubes through concentric needles, and are preferably focusedby a heated gas, e.g. heated air or heated inert gas. The rods areextruded into an environment that allows the solidification of the rodsprior to destabilization of the stable microjet. The fibers that areproduced can be used directly or drawn into a longer, thinner fiberdepending on the desired length and bandwidth of the optical fiber.Thus, the focusing technology can produced a long, thin optical fiberfor direct use in a cable or, preferably, the produced fiber can befurther drawn before use in an optical cable.

[0062] In a particularly preferred embodiment, the methods of thepresent invention are used to construct preforms for optical fibersbased on photonic bandgap structures. A photonic crystal is a structurethat repeats a structural element in one or more dimensions in space. Asa result of multiple reflections, certain wavelengths cannot propagatein these structures, and the structure is said to possess a ‘photonicband gap’ if it reflects a wavelength incident from any angle in space.Joannopoulos, J. D. et. al., Photonic crystals: molding the flow oflight, Princeton University Press, (1995); Cassagne D. et. al. Phys.Rev. B 52: R2216-R2220 (1995).

[0063] Initially, it was thought that a large contrast in refractiveindex would be needed to achieve a photonic band gap. An example of sucha contrast would be the refractive index between air and semiconductor,i.e. a refractive index, n, greater than 3. Studies have now shown thatit is possible to create a two-dimensional photonic band gap using amodest contrast in that refractive index between air and silica (n=1.5),providing the light has a component traveling parallel to the directionof the rods. Binks et. al., Electron. Lett. 31, 1941-1943 (1995).Wavelengths that are normally absorbed by silica can be transmitted formuch longer distances through air, and because air is not susceptible tothe nonlinear effects that occur in silica at moderate optical powers,much higher powers can be delivered using a photonic bandgap structure.Photonic band gap structures thus offer the ability to design newoptical properties into conventional materials by wavelength scaleperiodic micro-structuring of the material morphology.

[0064] Optical fiber preforms of this type are generally constructedusing multiple rods and/or tubes which are stacked to produce a desiredstructure. See e.g., Cregan et al., Science 285:1537-1539 (1999) andKnight et al., Science 285:1476-1478. Such tubes can be formed from anymaterial that allows the creation of the photonic bandgap structure,including but not limited to silica, glass, graphite, plastic and thelike. See Cregan et al., supra, and F. Gadot et al., Appl. Phys. Lett.71:1780 (1997). The structure is based on a defect in an otherwiseperiodic array of air holes placed within a honeycomb lattice.

[0065] For example, a number of solid silica rods of a constant diametercan be stacked horizontally to create a structure with a polyagonalcross-sectional structure. To create a waveguiding core within thestructure, a “defect” must be introduced into the crystal structure,i.e. a localized region with optical properties different from those ofthe fully periodic structure. This core is surrounded by a “cladding”,in this case the fully periodic region, which confines the light withinthe core. Preferably, a larger space is left in the center of thepreform to allow light to be guided down the central core. Theintroduction of extra air holes into the structure also can allowlocalized guided modes to appear within a band gap.

Photonic Bandgap Structures

[0066] There are two principle ways to reflect light at opticalfrequencies, total internal reflection (TIR) and reflection from aperiodic dielectric structure. TIR occurs at the interface between twodielectrics when it is not possible to simultaneously match both thefrequency and the phase on both sides of the interface. When light isincident from the high dielectric material, it is totally reflected backinto the material. This only occurs if the angle of incidence is greaterthan the critical angle. Light can also be reflected at the interfacebetween a homogeneous dielectric and a periodic dielectric. This occurswhen multiple scattered waves in the periodic medium destructivelyinterfere, thereby prohibiting propagation inside the periodic medium.

[0067] The interaction of light with glass now limits the maximum powerthat one can transmit with conventional glass optic fibers, which relyon TIR. Since no solid material has an index of refraction of less thanone, it is not possible to have hollow cores with fibers that rely onTIR because the core must have a larger index of refraction than thecladding.

[0068] A photonic crystal is a structure that repeats a structuralelement in one or more dimensions in space. As a result of multiplereflections, certain wavelengths cannot propagate in these structures,and the structure is said to possess a ‘photonic band gap’ (PBG) if itreflects a wavelength incident from any angle in space. Joannopoulos, J.D. et. al., Photonic crystals: molding the flow of light, PrincetonUniversity Press, (1995); Cassagne D. et. al. Phys. Rev. B 52:R2216-R2220 (1995). Structures having such photonic crystals, and thuspossessing PBGs, can be used as optical fibers since they have theability to direct light through hollow portions of the structure thatare enclosed by material possessing an appropriate photonic bandgap.

[0069] In one embodiment, the methods of the invention can be used toproduced optical fibers composed of PBG structures. As mentioned above,these structures allow propagation of light through a hollow corewithout certain limitations found in fibers utilizing TIR, e.g. therequirement for a higher index of refraction of the cladding. The use ofair as the medium though which light travels also prevents theabsorption of certain wavelengths that are absorbed by core materials(e.g., glass) of conventional optical fibers. It is possible using themethods of the present invention to make PBG structures which allowsingle mode transmission of light along a hollow structure in a fiber.

[0070] Initially, it was thought that a large contrast in refractiveindex would be needed to achieve a photonic band gap. An example of sucha contrast would be the refractive index between air and semiconductor,i.e. a refractive index, n, greater than 3. Studies have now shown thatit is possible to create a two-dimensional photonic band gap using amodest contrast in that refractive index between air and silica (n=1.5),providing the light has a component traveling parallel to the directionof the rods. Binks et. al., Electron. Lett. 31, 1941-1943 (1995).Wavelengths that are normally absorbed by silica can be transmitted formuch longer distances through air, and because air is not susceptible tothe nonlinear effects that occur in silica at moderate optical powers,much higher powers can be delivered using a photonic bandgap structure.Photonic band gap structures thus offer the ability to design newoptical properties into conventional materials by wavelength scaleperiodic micro-structuring of the material morphology.

Construction of the Optical Fiber Preform

[0071] A variety of optical fibers are known, and each of these fiberscan be produced from a specific preform. In a preferred embodiment ofthe present invention, the optical fiber preforms are composed ofphotonic crystals containing periodic regions that create PBGs. Opticalfiber preforms of this type are generally constructed using multiplerods and/or tubes which are stacked to produce a desired structure. Seee.g., Cregan et al., Science 285:1537-1539 (1999) and Knight et al.,Science 285:1476-1478 (1998). Such tubes can be formed from any materialthat allows the creation of the photonic bandgap structure, includingbut not limited to silica, glass, graphite, plastic and the like. SeeCregan et al., supra, and F. Gadot et al., Appl. Phys. Lett. 71:1780(1997). The structure is based on a defect in an otherwise periodicarray of air holes placed within a honeycomb lattice.

[0072] For example, a number of solid silica rods of a constant diametercan be stacked horizontally to create a structure with a polyagonalcross-sectional structure. To create a waveguiding core within thestructure, a “defect” (or hollow section) must be introduced to achieveoptical properties different from those of the fully periodic structure.This core is surrounded by a fully periodic region, which acts as a“cladding” and confines the light within the core. Preferably, a spaceis left in the center of the preform to allow light to be guided downthe central core. Alternatively, multiple hollow spaces may be leftwithin the periodic structure to allow light to travel down multiplechannels. The introduction of extra air holes into the structure alsocan allow localized guided modes to appear within a band gap.

[0073] In another example, a exemplary single mode optical fiberconsists of a core of 10 μm diameter in the center, a cladding of 125 μmdiameter surrounding the core, and a protective jacket formed by resincovering the cladding. The optical fiber glass preform is consequentlyalso comprised of an inner core portion and a cladding coating. Thisoptical fiber glass preform itself is conventionally formed byconverting a soot body for forming the optical fiber porous glasspreform into transparent glass.

[0074] Conventional methods of producing optical fiber porous glasspreforms include the OVD method (outer deposition type CVD method) andthe VAD method. Preforms for use with the present invention can beproduced using these or other methods of creating preforms that areknown to those skilled in the art.

[0075] For example, the general method of production of an optical fiberporous glass preform using the VAD method involves preparing a seed bar(hereinafter referred to as a “target bar”) and placing the target barinside a reaction container, a reaction chamber with one end suspendedfrom an upper side so that the target bar can be rotated around itslongitudinal center axis. Oxygen, hydrogen, and other combustion gasesand the SiCl₄ glass particle material (including a dopant such as GeCl₄if desired) are fed to oxyhydrogen burners from which oxyhydrogen flamesare generated. In the oxyhydrogen flames formed by the combustion gasesfrom the burners, the moisture in the oxyhydrogen flames and the SiCl₄undergo a hydrolysis reaction as shown by the following reaction formulaand form SiO₂, which is the main component of the glass particles:

SiCl₄+2H₂O=SiO₂+4 HCl

[0076] These glass particles are sprayed to the lower part of therotating target bar and deposited thereon to form the optical fiber sootbody.

[0077] The optical fiber soot body formed by the VAD method is thenconverted to transparent glass to form the optical fiber porous glasspreform used for producing an optical fiber. Note that an optical fibersoot body converted to transparent glass can further have glassparticles deposited around it, if necessary. In this case, afterdepositing the glass particles, the soot body is again converted totransparent glass to form the optical fiber glass preform.

[0078] Other methods for creating conventional optical fiber preformsare described in U.S. Pat. Nos. 4,224,046; 4,419,116; 4,421,540;5,320,660;

[0079]5,397,372; 5,672,192.

Coating the Optical Fibers

[0080] Optical fiber cables containing a plurality of optical fibers forthe transmission of optical signals are well known. Such optical fibercables typically include a core which may have a strength member tocarry the axial tensile stress and axial compressive forces on thecable. Also located within the core are one or more tubes. Each tubetypically includes a plurality of optical fibers. The optical fiberswithin a tube may be individually stranded or may be provided in anoptical fiber ribbon. A sheath is provided to enclose the core includingthe tubes and the strength member. The optical fibers included withinsuch a cable typically include a glass core and one or more claddingsand/or coatings.

[0081] During a process of manufacturing a glass optical fiber, a glassfiber is drawn from a preform and then coated with one or more coatingmaterials, typically ultra-violet light curable materials. The coatingmaterials include, for example, polymeric compositions and are appliedby one or more coating applicators. The function of the fiber coating isto protect the surface of the glass optical fiber from mechanicalscratches and abrasions which the optical fiber may experience duringsubsequent handling and use. The coating or coatings also influence thefiber's optical characteristics in response to external mechanicalforces and environmental temperature.

[0082] Optical fibers are almost universally color-coded in their enduse. There are numerous colors which are acceptable in most markets,with additional identification being made possible by “banding” coloredfibers with additional colors or circumferential striping. Onewell-known method of coloring an optical fiber is to apply an ink layerto an optical fiber having single or dual coating layers so that thetotal composite optical fiber includes primary and secondary coatinglayers with an outermost ink layer. The ink coloring layer is thin,typically 3 to 5 microns in thickness, and typically includes a carrierresin and a pigment system. The carrier resin may typically be a solublethermoplastic material or a ultra-violet (UV) curable resin. In theformer, the ink is applied via a dye or a transfer method, such as afelt-tip applicator or roller, and the solvent for the carrier resin isdriven off by heat to leave the pigmented resin on the fiber. In the UVsystem, there is no solvent. The liquid resin pigment is cured to asolid state by UV energy. Either ink involves a separate step fromeither optical fiber production or the cabling operation.

[0083] An alternative method for color-coding the fiber is to have thecolor mixed directly into a secondary (outer) coating of a dual coatedoptical fiber. The secondary coating acts as the carrier resin for thecoloring agents.

[0084] In one embodiment, the optical fibers are coated during thefocusing procedure using the desired liquid coating as the surrounding,focusing fluid. For example, a preform can be heated and focused usingan outside liquid composed of a liquid resin pigment. As the preform isfocused, it is also coated by the liquid resin and pigment, and uponexpulsion of the focused optical cable it would retain an outer coatingof the focusing material. The focused optical fiber is then expelledinto a gaseous environment and immediately cured to a solid state usingUV energy.

Preform and Fiber Characteristics

[0085] The methodology of the present invention can be used to produceboth preforms which can then be used via conventional drawing technologyto produce fibers or, alternatively, can be used to produce fibers frompreforms produced using conventional technology or alternatively thepresent invention can be used to produce fibers using preforms producedaccording to the methodology described herein. Preforms which have aconstant diameter along their length can be produced using thetechnology described herein particularly wherein the diameter variesalong the length of the fiber or preform from ±1% or less to as much as±30% or less. Further, fibers can be produced wherein the diameter alongthe length of the fiber is substantially constant for example an opticalfiber of silica glass can be produced having a diameter of about 1micron wherein the diameter along the length of the fiber varies ± about10% or less or more preferably ± about 1% or less.

[0086] The methodology of the present invention provides fiber formingstability which may be enhanced by the appropriate external pressuredistribution provided inside of the oven or pressure chamber. The stresson the fiber may be dramatically reduced since the glass-to-solidcontact is avoided by the surrounding focusing fluid or gas shroud.Complex fiber concentric structures may be formed by the dramaticreduction of the radial viscous stresses compared to the classicalextrusion or drawing technology. Molten or semi-molten/semi-solid fibersextruded from the nozzle of the invention can be combined together toprovide phonic bandgap structures of any desired configuration. Not onlyare the characteristics of such photonic bandgap structures and fibersproduced via the present technology desirable but the processing itselfresults in desirable characteristics such as avoiding clogging of theextrusion device due to the surrounding focusing fluid and avoidingcontamination of the fiber material due to contact with such solidobjects. Still further, the fiber quality may be enhanced by the rapidfiber quenching which occurs due to the expansion of the gas exiting thenozzle.

EXAMPLES

[0087] The following examples are put forth so as to provide those ofordinary skill in the art with a complete disclosure and description ofhow to make and use the present invention, and are not intended to limitthe scope of what the inventors regard as their invention nor are theyintended to represent that the experiments below are all or the onlyexperiments performed. Efforts have been made to ensure accuracy withrespect to numbers used (e.g. amounts, temperature, etc.) but someexperimental errors and deviations should be accounted for. Unlessindicated otherwise, parts are parts by weight, molecular weight isweight average molecular weight, temperature is in degrees Centigrade,and pressure is at or near atmospheric. Units of pressure describedherein are assumed to be Pascals units and viscosity is in kilograms permeter per second, length as in meters and velocity is in meters persecond.

Example 1 Focusing of a Conventional Optical Fiber Preform

[0088] Optical fibers are typically manufactured by constructing anoptical fiber preform of appropriate composition and drawing fiber fromthe preform. A preform is a concentric glass rod having a length ofabout one meter and an outer diameter of 20 mm. The inner core of therod is a high purity, low loss glass such as germanium silicate glasshaving a diameter of about 3 mm. The concentric outer cylinder; referredto as cladding, is a layer of glass with a lower index of refractionthan the inner core.

[0089] Drawing of this preform is performed using a device asillustrated in FIG. 1. Briefly, the end of the preform serves as adelivery source which is heated to a temperature that allows the preformmaterial to become ductile. In general, high viscosity liquid fiberslike the silica require working in the temperature range 700-1000° C.(about 1000 to 1300° K), with viscosities ranging from 1,000,000 to1,000,000,000 cpoises. The preform is kept in the heated environment fora sufficient time to become ductile, and then is introduced from thepreform delivery source into the pressure chamber or oven with thesurrounding focusing gas. The preform is introduced into the pressurechamber by opening the preform delivery source using the force ofgravity and optionally a pulse of heated gas into the delivery sourcechamber. The focusing gas is introduced into the pressure chamber via asecond entry port. The focusing gas may also be heated to maintain theductility of the preform.

[0090] The focusing fluid or gas within the oven 3 rushes towards theexit 4 of the nozzle 5 creating a pull on the highly viscous glass ofthe preform and tube. Thus, the cylindrical structure of the preform isincreased in length and decreased in a cross-sectional dimension as itmoves toward the nozzle 5. Within the nozzle 5 additional forces arecontinually applied along the length “L” of the nozzle until it exitsthe opening 4 of the nozzle 5 and is cooled by the rapid expansion ofthe gas also exiting the opening 4 into the atmospheric pressure P_(a).When the fiber 7 exits the opening 4 of the nozzle 5 it is moving at avelocity “V₁” which is substantially faster than the velocity “V₀” whichit is moving at when it is at the end 2 of the preform 1. Further, thegas is expanding supersonically for λ>0.635.

[0091] In accordance with the invention it is possible to repeatedlyapply the methodology as shown within FIG. 1. Specifically, the fiber 6exiting the opening 4 could be the beginning of the same processingsteps whereby that length of viscous material enters and oven 3′ andenters a nozzle 5′ in order to be further stretched, i.e. increased inits longitudinal dimension and decreased in its cross-sectionaldimension so as to produce a fiber of any desired length or dimension.

[0092] Because of the different physical phenomenon and the scaling lawsfor high viscosity liquids, the nozzle is preferably designed to undergothe extremely high pressures needed to draw the fiber 6 by the gas jetthrough the 200 microns nozzle. For example, certain fluids may exhibita violent flapping or ‘whipping’ instability upon exiting the chamber.This can be accounted for in the design of the device and in the mannerin which the flow is expelled. For example, the flow and the fiber candischarge into a vacuum chamber so that the fiber 6 will flash-cool andwill undergo a favorable pressure gradient along the vacuum chamber.

Example 2 Focusing of a Photonic Bandgap Optical Fiber Preform

[0093] An optical fiber composed of one or more photonic bandgapstructures can be drawn using the method and devices of the presentinvention.

[0094] Optical fibers based on photonic bandgap structures have beenrecently described in the art. These structures have a photonic bandgapstructure as the cladding, which forces light to remain in the hollowcore. The structure is constructed from silica tubes and rods, as thecontrast between silica and air (1.46 to 1) has been shown to besufficient to produce a bandgap useful for these purposes. Cregan etal., supra and Knight et al., supra. The bandgap created depends uponthe geometrical arrangement of the preform, and altering structuralaspects such as the holes, the size of the holes and the distancesbetween the holes will also alter the bandgap created. The PBG structurefor use as an optical fiber is preferably a structure with a largervolume of air in the center, e.g. from about 25-45%, and preferablyaround 30%.

[0095] All fibers can be produced using two basic different methods. Inaccordance with the preform 1 (as shown within FIG. 1) may be a hollowcylinder. In accordance with an alternative configuration the moltensilica glass is provided by a tube which extrudes molten silica andwhich tube is concentrically positioned around a second tube whichsimultaneously extrudes a gas and maintains the gas at a pressure whichis substantially the same as the pressure “P₀” within the oven orpressure chamber 3. It is possible to align a plurality of componentssuch as shown in FIG. 1 so that they simultaneously extrude hollowsemi-solid tubes which come into contact with each other while still ina semi-solid state. The tubes can then be made to fuse to each other andform a photonic bandgap structure when the different tubes are correctlypositioned in a manner known to those skilled in the art.

Example 3 Focusing of Elements of a Photonic Bandgap Optical FiberPreform

[0096] In addition to or in combination with the focusing of Example 2,precursor elements of the optical fiber PBG preform can be focused priorto construction and/or fusion of the preform. The PBG preform areconstructed using rods and/or tubes that are bundled to create certaineven spacing of holes within the structure.

[0097] Hollow silica or plastic rods are provided, and then modifiedusing the focusing methods of the present invention. These tubes can befocused to a very specific and small diameter using the methods of thepresent invention, and the focused tubes can be used to produce anoptical fiber preform composed of such tubes. The liquid flow is thenfocused into a microjet by a gaseous outer fluid, and the tubes areexpelled into a gaseous environment where they solidify and arecollected.

[0098] To construct the preform, several hundred of the hollow, focusedtubes are bundled into a hexagonal array. The diameter of the tubes usedto construct the preform depend upon the desired size of the holes inthe bandgap, as will be apparent to one skilled in the art upon readingthe present disclosure. The diameter of the tubes will control thespaces between the tubes and the resulting periodic space in the opticalfiber made from the preform. Once the tubes have been stacked, anappropriate number of tubes (e.g., 5-50 tubes) are removed to provideone or more hollow cores through which light can travel.

[0099] Following construction, the preform can be fused and prepared tobe drawn into a fiver. Alternatively, the unfused preform can besubjected to an elongation event such has that described in Example 1,and fuse either during following said event.

Mathematical Formulation

[0100] We consider a Newtonian viscous liquid concentrically drawnthrough a convergent-divergent micro-nozzle with length L, andsurrounded by a high speed gas stream as sketched in FIG. 1, assuming anaxisymmetric configuration (the effect of asymmetries is describedbelow). The object is to obtain a fiber of final radius a <<L at a givenvelocity V₁. We define non-dimensional variables x, f, v, and p standingfor the axial coordinate, fiber radius, liquid velocity, and liquidpressure, which are made dimensionless with L, α, V₁, and 3 μ_(o)V₁/L,where μ_(o) is a reference liquid viscosity. Furthermore, owing to thedisparity in the residence times of particles of the liquid and the gas,the gas can be considered steady for any non-steady liquid motion ofinterest (including motions with wavelengths of the order of the fiberdiameter). The gas pressure and temperature can be considered as steadyvariables in the problem, which are functions of the axial coordinateonly.

[0101] For the sake of generality, we can assume a non-linear dependenceof the liquid viscosity with temperature μ=μ(T₁), where T₁ is the liquidtemperature. Using viscous liquids at temperatures around a “drawing”point given by T_(o) for which the liquid viscosity is μ_(o) (μ_(o)=10⁵Pa·s in many practical situations), the relationship between viscosityand temperature is well fitted by the law μ=μ_(o)10^(−κ(T)l^(/T) _(o)⁻¹), where κ is a non-dimensional constant (in the case of commonglasses used in fiber optics, we have T_(o)˜1000K and κ about 10 to 15).One can then define a non-dimensional viscosity ζ=μ/μ_(o). Thus,neglecting gravity forces, liquid acceleration and surface tensionforces compared to the much larger viscous and pressure forces, one canwrite the well known mass and momentum conservation equations for theliquid as:

_(t) f ²+._(x)(f ² v)=0; _(x) p= _(x)(ζf ² _(x) v)/f ² +f _(s)  (1)

[0102] with boundary conditions (i) p=p_(o)=(LP_(o))/(3 μV₁), f=E^(1/2),and v=E⁻¹ at x=0, and (ii) v=1 at x=1, where E=V_(o)/V₁, V_(o) is theintake liquid velocity (or the glass preform velocity in glass fiberdrawing). f_(x)=F_(s)/(3 μV¹/L²), where Fs˜τ/Ε is the axil resultant perunit volume owing to the viscous stress on the surface, and τ is theviscous stress on the liquid surface owing to the gas stream. Assuming ahigh viscosity liquid, the fiber radial velocity profile is almostperfectly flat since the viscous diffusion time t_(v)˜ρα²μ⁻¹ is manyorders of magnitude smaller than the hydrodynamic time t_(o)˜LV₁ ⁻¹(i.e., μL (ρV₁α²)⁻¹<<1).

[0103] On the other hand, the gas flow is governed by the well knownisentropic compressible 1-D, N-S equations: the gas pressure andtemperature distribution are given by their stagnant values P_(o) andT_(o), respectively, and the nozzle geometry through A(x), its localcross section area. The gas expansion in the nozzle provokes a change inthe gas temperature along the nozzle which is given by

T/T _(o)=1−ΔT/T _(o)=(1−ΔP/P _(o))^((γ-1)/γ)  (2)

[0104] in the isentropic assumption, where γ=C_(p) ^((g)/C) _(v) ^((g))is the adiabatic gas constant, C_(p) ^((g)) and C_(v) ^((g)) v are theusual gas heat coefficients at constant pressure and density,respectively, and ΔP is the pressure drop at a certain point of thenozzle from its entrance. This can be approximated by ΔT≃(γ−1)γ⁻¹ΔPP_(o)⁻¹T_(o)).

[0105] Assuming a slender fiber, its local temperature profile T1 isgiven by

v _(x) T ₁=α _(r)(r _(r) T ₁)/r(  (3)

[0106] with boundary conditions T₁(x; f)=T_(s)(T₁(x; 0) ), and T₁(0,r)=T_(o), where T_(s) is the gas temperature at the fiber surface, r isthe radial coordinate (made dimensionless with α) and aα=K/(v₁ρC_(ρ)α²). P, K and C_(ρ) are the liquid density, thermalconductivity and heat capacity, respectively. We can distinguish twolimiting problems:

[0107] 1. a>>1: In this case, we can assume T₁=T_(s). We call this the“gas limited” (GL) case.

[0108] 2. a<<1: In this case, we can assume T₁=T_(o). We call this the“isothermal” (IT)limit.

[0109] In the GL case, the temperature profile of the fiber in theradial direction can be considered uniform, and the temperature iscontrolled by the ability of the gas to trans-port the heat through itsthermal boundary layer. In the other limit, IT, the liquid bulk remainsat the initial T_(o) temperature because the inability of the liquidthermal boundary layer to evacuate the liquid heat.

[0110] From the point of view of the fiber shape homogeneity andquality, the GL case is the most interesting one, because it involves acontrolled temperature (i.e., material quenching and enhanced amorphousstructure) and a substantial increase in the fiber viscosity (andtherefore an increase in its “mechanical resistance”) as it proceedstowards the nozzle exit, which immediately suppresses most instabilitiesby itself without the need of a further refinement of the process,although it requires a limited production velocity, given byV₁<<KL/(ρC_(ρ)α²). These velocities can be accomplished by the classicalsimple hot drawing process. in some cases, but the temperature controlis in this process severely limited.

[0111] On the contrary, the IT case is challenge because it is mostunstable and difficult to control, but its reward is its largeproductivity (large E values). Because of this, we will focus on thisparticular limit, also considered by Yarin et al. (1999) in the case ofsimple drawing. We will show that the use of the co-flowing high speedgas stream provide the means to (i) completely stabilize the fiber, to(ii) yield fiber homogeneity and/or shape control, and to (iii) controlfiber quenching. Although we are interested in the IT case, for the sakeof generality in the following we will consider the temperaturevariations in the liquid for the GL limit also.

[0112] Gas boundary layer, viscous shear stress, and heat transfer onthe fiber surface—The gas boundary layer on the liquid jet has athickness of the order of ε˜O((μ_(g)LP_(o) ⁻¹)^(1/2)(R_(g)T_(o))^(1/4)), where P_(o), T_(o), and μ_(g) are the stagnationgas pressure and temperature at the nozzle entrance, and ¹g is the gasviscosity, respectively, and R_(g)=C_(ρ) ^((g))−C_(v) ^((g)) as usual.The tangential viscous stress τ acting on the jet surface, owing to themuch faster gas stream, is then of the order ofτ˜O((μ_(g)L⁻¹P_(o))^(1/2) (R_(g)T_(o))^(1/4). Comparing the axialresultant per unit volume of the viscous stress on the surface, F_(s),of the order of F_(s)˜τ/α, with the extensional (axial) resultant of theviscous stress, F_(v), of the order of F_(v)˜μV₁/L₂, and sinceP_(o)˜μV₁/L, one obtains

F _(s) =F _(v)˜(μ_(g)/μ₁)^(1/2) (R _(g) T _(o) /V ₁ ²)^(1/4) L/α(  (4)

[0113] We seek for production velocities V₁ much larger thanμ_(g)μ⁻¹L²α⁻²(R_(g)T_(o))^(1/2) (of the order of about 10⁻³ to 10⁻² m/sin practical situations), for which F_(s)<<F_(v), and the contributionof the surface stress is negligible versus the axial component of thenormal pressure stress, of the order of P_(o)L⁻¹ ˜μV₁L⁻². Thus, themomentum equation in (1) reduces to _(x)ρ=∂ _(x)(ζf² _(x)V)/f².

[0114] Since the gas temperature variations can be approximated byequation (2), considering a portion of the threadline, its temperaturevariation owing to the heat transfer through the gas thermal boundarylayer (of the order of ε since the gas Prandtl number is of the order 1)is of the order of${\Delta \quad T_{1}} \sim {\frac{L}{\rho \quad C_{p}\alpha \quad {V1}}\kappa_{o}\frac{\Delta \quad T}{\delta}}$

[0115] where κ_(o) the gas thermal conductivity at temperature T_(o). Inpractical situations, we may have L(ρC_(ρ)αV¹)⁻¹κ_(o)ε⁻¹<<1; however,owing to the strong dependence of viscosity with temperature, theselimited liquid temperature variations (about 5 to 20% of T_(o) inpractice).are sufficient to increase the liquid viscosity by orders ofmagnitude, which is a mechanism that suppresses most instabilities byitself.

[0116] It should be discussed here that owing to the high P_(o) valuesneeded to drive the fiber, the gas undergoes its maximum expansion andconsequently its maximum cooling right after the nozzle exit. In the GLcase, the fiber is sufficiently hardened to remain unaffected by thisexpansion. In the IT limit the nozzle exit geometry and the fiberwinding system after the exit should be carefully designed to avoidfiber shape inhomogeneities.

[0117] Nozzle geometry—In order to reduce the problem of the nozzlegeometry to a single parameter, without lost of generality on our aimedtask, we have selected pressure distributions of the type:

ρ(x)=ρ_(o) e ^(−λx)

[0118] where λ is a free parameter, and the set of parameters {ρ_(o), λ}will be optimized for the requirement of an unlimited fiber drawing(i.e. fiber production) with a minimum energy consumption (minimumρ_(o)). Thus, given stagnation pressure ρ_(o), we seek λ values forwhich the drawing is absolutely stable for any (unlimited) given“productivity” E value.

[0119] Fiber stability. Suppression of the non-symetric in-stability(fiber whipping)—Considering ρ_(o) values of the order unity, for valuesof λ<0.635, one obtains a sub-sonic gas flow along the nozzle, except atthe nozzle exit. Calculating the pressure distribution on the fiber whenit undergoes a small departure from the axisymmetric configuration (werecall that gas and liquid flows are uncoupled with time, and that thegas flow can be calculated in steady regime), one obtains a strongdestabilizing effect. On the contrary, for λ values sufficiently largerthan 0.635, the resulting supersonic part of the nozzle flow provokesthe reversal effect: any departure from the basic axisymmetricconfiguration provoke a strong re-aligning azimuthal pressuredistribution, which suppress any possible incipient fiber whipping.

[0120] Thus, we will consider λ>1 values only in our analysis.

[0121] Suppression of the “drawing resonance” axisymmetric instability.IT case—Consider the small perturbations problem given by

F=f _(e)(1+αe ^(ΛT)), v=v _(c)(1+βe ^(Λt))

[0122] and governed by equations (1), where f_(e)(x) and v_(e)(x) arethe steady values of the problem for the given boundary conditions, α(x)and β(x) are complex functions with argument small compared to 1, and Λis the complex perturbation growth rate. Thus, α and β are governed by aset of 2 complex ODEs with homogeneous boundary conditions that can bereadily obtained from equations (1) (problem not written here forsimplicity; see [1]), which determine the eigenvalue Λ, and whose realpart Λ_(r) gives the growth factor in time. We give the value Λ_(r) as afunction of ρ_(o) for λ=6and E=120.

[0123] For every given E and λ values, there is a corresponding ρ_(o)value above which any axisymmetric instabilities are suppressed. In FIG.3A we plot the curves which divide the {E, p_(o)} space into stable(above the curve) and unstable (below the curve) parametricalsub-spaces, for several λ values of practical interest. One mayimmediately note from this plot that for every given λ value, there is aparticular limiting value of ρ_(o) for stabilization above which thefiber is stable for any value of E>1. In FIG. 4, we plot these limitingρ_(o) values as a function of λ, and obtain a universal minimum value ofthe pressure above which the fiber is absolutely stable regardless thefiber productivity E.

[0124] A minimum limiting ρ_(o) value about 0.33 around the point λ≃5.65can be found, corresponding to the optimum nozzle shape with asignificant supersonic region (from x=0:112 to x=1) for a singleparameter geometry. In this case, given a desired fiber diameter a andfiber production velocity V₁, the minimum gas pressure P_(o) necessaryto have absolute stability is given by the simple expression:

P _(o) =μV ₁/α  (8)

[0125] with a nozzle shape as shown in FIG. 2.

[0126] A practical case—Consider the production of 173 Km/day of a fiberwith a diameter of 200 μm of an alumino-silicate glass (μ_(o)=10³ Pa·sat T_(o)=1500K). The minimum pressure required is then P_(o)=2×10 Mpa,with a nozzle exit pressure of P_(s)=P_(o)×e^(−5.65)=70.3 Kpa. Assumingthat the nozzle exits into an adapted pressure chamber, at a temperatureT_(s)=300 K, the isentropic compression of the gas from the exit chamber(recirculation) rises the temperature to the required T_(o)≃1500K. Theliquid has density, thermal conductivity and heat capacity ρ=3000 Kg·m³,K=1 J(m·s·K)⁻¹, and C_(p)=1000 m² (s²·K), respectively. The nozzledesign should minimize the neck-fiber gap with-out compromisingstability. A feasible solution is a nozzle with a length of 10 mm, aneck diameter of 1.3 mm at 1.12 mm from the entrance, and exit diameterof 1.28 mm. Considering the presence of the fiber, this nozzle has aminimum (sonic) cross section area of 0.32 mm² . Since V₁=2m·s⁻¹>>KL/(ρC_(ρ)α²)=0.3 m·s⁻¹, the fiber can be consideredquasi-isothermal (IT limit). The minimum theoretical power consumptionof the plant is then W=9.54 KW, with a gas flow rate of about 1 l/s atPs=0.7 Bar. Scale-up is straightforward.

[0127] A further refinement of the nozzle geometry is possibleintroducing new geometrical parameters (the minimum ρ_(o) value may befurther optimized). This refinement does not limit the generality of theabove analysis.

[0128] [1] A. L. Yarin, P. Gospodinov, O. Gottlieb, M. D. Graham, Phys.Fluids 11, 3201 (1999).

[0129] While the present invention has been described with reference tothe specific embodiments thereof, it should be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims appended hereto.

1-28. (cancel).
 29. A method of producing a fiber from a cylindricalpreform, comprising the steps of: heating a first end of a cylindrical,ductile, preform in a pressure chamber wherein the preform has a firstlateral dimension along a longitudinal axis; applying physical pressureto the heated first end by flowing a focusing gas along a portion of thelongitudinal axis of the preform, wherein the focusing gas is caused toflow in a direction along the preform and toward the first end of thepreform so that the first end of the ductile preform is accelerated bytangential sweeping forces exerted by the focusing gas thereby forming afiber with a decreased lateral dimension relative to the first lateraldimension of the preform.
 30. The method of claim 29, wherein thefocusing fluid is heated.
 31. The method of claim 29, wherein thepreform is comprised of silica glass.
 32. The method of claim 29,wherein the preform is an optical fiber preform comprised of silica. 33.The method of claim 29, wherein the cylindrical preform is a solidcylinder comprising silica glass and is expelled from the exit openingas a solid cylindrical fiber.
 34. The method of claim 29, wherein thecylindrical preform is a hollow cylinder comprising silica glass and isexpelled from an exit opening of a pressure chamber as a hollowcylindrical fiber.
 35. The method of claim 29, wherein the gas is heatedinert gas and the gas exits an exit opening of a pressure chamber atsupersonic speed.
 36. The method of claim 29, wherein the ductilepreform is drawn through a nozzle which nozzle begins as an openinginside a pressure chamber and extends along a curved surface, ending atan exit opening of the pressure chamber.
 37. The method of claim 36,wherein the curved surface of the nozzle has a surface configurationwith a nozzle parameter geometry defined by an equation p(x)=p ₀ e^(−λx) where p(x) is a curve defining function which plots the nozzlegeometry, ρ₀ is the internal pressure of the focusing gas as it entersthe nozzle, λ is greater than 0.635 to obtain supersonic speed for thefocusing gas and x is a function.
 38. The method of claim 37 where λ is2.0 or more.
 39. The method of claim 37 where λ is about 5.65.
 40. Themethod of claim 36, wherein the equation$P_{0} \geq \frac{\mu_{l}V_{1}}{L}$

applies and P₀ is the pressure at an entrance port to the pressurechamber; μ_(l) is the viscosity of the ductile preform end, V₁ is thevelocity of the fiber in the nozzle and L is the length of the nozzle.41. A method of producing a fiber from a molten viscous liquid;comprising the steps of: extruding a stream of a molten viscous silicaglass in a manner so as to flow from a supply source into a pressurechamber wherein the stream has a first circumference; supplying afocusing gas to the pressure chamber whereby the gas enters through anentrance port of the pressure chamber and exits through an exit port ofthe pressure chamber positioned downstream of the flow of the stream ofmolten viscous silica glass; wherein the gas surrounds the stream ofmolten viscous silica glass and compresses the first circumferencecreating a narrowed stream of a second circumference narrower than thefirst circumference, which narrowed stream is expelled from the exitport of the pressure chamber as a fiber of silica glass.
 42. The methodof claim 41, wherein the gas is a heated inert gas and the gas exits theexit opening of the pressure chamber at supersonic speed.
 43. The methodof claim 42, wherein the stream of molten silica glass flows through anozzle which nozzles begin as an opening inside the pressure chamber andextends along a curved surface ending at the exit port of the pressurechamber.
 44. The method of claim 43, wherein the curved surface of thenozzle has a surface configuration with a nozzle parameter geometrydefined by an equation p(x)=p ₀ e ^(−λx) where p(x) is a curve definingfunction which plots the nozzle geometry, ρ₀ is the internal pressure ofthe focusing fluid as it enters the nozzle, λ is greater than 0.635 toobtain supersonic speed for the focusing fluid and x is a function. 45.The method of claim 44 where λ is 2.0 or more.
 46. The method of claim44 where λ is about 5.65.
 47. The method of claim 44, wherein theequation $P_{0} \geq \frac{\mu_{l}V_{1}}{L}$

applies and P₀ is the pressure at an entrance port to the pressurechamber; μ_(l) is the viscosity of the ductile preform end, V₁ is thevelocity of the fiber in the nozzle and L is the length of the nozzle.48. A method of producing an optical fiber preform element, comprisingthe steps of: providing a hollow tube having a longitudinal axis;applying physical pressure to force the tube through a feeding source ina manner which causes the preform to be expelled from an exit opening ofthe channel in a longitudinal manner; and forcing a fluid through apressure chamber in a manner which causes the fluid to exit the pressurechamber from an exit orifice in front of a flow path of the preformexpelled from the exit opening of the channel, wherein the fluidsurrounds said preform and focuses said preform in a longitudinal mannerto expel an optical fiber from said pressure chamber.
 49. A device forproducing a fiber, comprising: a pressure chamber comprising an entranceport for adding a focusing fluid and an exit port for expelling aviscous liquid; and a nozzle positioned in the exit port, the nozzlecomprising a curved surface with a geometry defined by an equationp(x)=p ₀ e ^(−λx) where p(x) is a curve defining function which plotsthe nozzle geometry, ρ_(o) is the internal pressure of the focusingfluid as it enters the nozzle, λ is greater than 0.635 to obtainsupersonic speed for the focusing fluid and x is a function.